The use of outside air to reduce mechanical cooling requirements of building ventilation and air conditioning systems is well known in the art and has been an integral part of ventilation system design involving air handler units (AHUs) or packaged rooftop units (RTUs), hereafter referred to as AHUs.
Regardless of geographic location, most commercial buildings require cooling during each season, due to the large heat gains realized within a typical facility, and this cooling requirement can have a significant impact on operating costs. In many geographic locations, however, especially during the cooler or more temperate months of the year, these cooling needs can be offset by way of an airside economizer. An airside economizer (hereafter referred to as an “economizer”) is a system that typically incorporates a linked damper assembly (mechanically connecting together the outside, return, and building exhaust dampers common to an air handler) or dampers that are controlled in unison through other control means, and control logic that determines the amount of outside air to introduce in order to reduce mechanical cooling needs. If done properly, the result can lead to substantial energy savings. Given these benefits, ASHRAE standard 90.1-2010 (“Energy Standard for Buildings Except Low-Rise Residential Buildings”) lists the airside economizer as a conditional requirement (assuming a water-side economizer is not specified) for systems having a cooling capacity of 54,000 Btu/h or more.
In order to determine when conditions are suitable for operating the economizer, a means of sensing the “heat” or energy level present in the outside air and, often times, return air is applied which, as known in the art, may involve one of several control strategies including: fixed dry-bulb temperature measurement, dual or differential dry-bulb measurement, fixed enthalpy measurement, dual or differential enthalpy measurement, or combinations of these measurement strategies. Each of these approaches is well known to those of ordinary skill in the art. In particular, well known conventional techniques involve measuring the enthalpy of the AHU return air and comparing it to that of the outside air. Using this technique, employing this“differential enthalpy” measurement, the economizer is enabled when the return air enthalpy exceeds that of the outside air. An example of a commercially available sensor product used for this purpose includes the Honeywell C7400A1004.
U.S. Pat. No. 4,362,026, which is incorporated herein by reference, is a further example of the use of an enthalpy measurement to provide what is known as a “lockout” or “changeover” function. In general, systems and methods used to assess when it is appropriate to enable an economizer are commonly referred to as economizer high limit controls. Still further, U.S. Pat. No. 4,182,180, which is incorporated herein by reference, discloses an approach to measuring enthalpy.
In addition, the lockout function or high-limit controls may include additional lockout settings that automatically disable the economizer when the outdoor air dry-bulb temperature goes above or below pre-determined values in order to provide freeze protection when outdoor conditions become very cold and, for fixed dry-bulb or differential dry-bulb economizers, to prevent the introduction of air that is too moist at higher temperatures.
When a system is equipped with an economizer, savings can be realized on the cooling costs associated with the load posed by return air, because the economizer acts by increasing the outdoor air percentage to the building when it requires less energy, and therefore is more economical to use the outdoor air in lieu of the return air. In essence, an economizer incorporates a damper system that controls outside air and return air in an inverse fashion: as the outside air volume (flow rate) is increased the return air volume is decreased by the same amount.
FIG. 1 illustrates the heating and cooling operation of a typical prior art AHU with an economizer which generally involves four functional modes that are dependent on the thermal demand for heating or cooling and the suitability of using outside air for cooling purposes.
In region “A” the outdoor air temperature is below a point where, for the minimum setting of the outdoor air damper the resultant air mixture at the intake of the Air Handler Unit (AHU) has to be heated in order to realize the desired AHU discharge air temperature (typically 55° F.). The operating state of the system in this region, generally involves an active heating coil at the AHU. The boundary between Region A and Region B is often referred to in the art as the pivot point temperature of the building, and is the normal point at which the outdoor air temperature is sufficiently low to require heat to be added at the AHU. This is the case whether there is an economizer or not. Notice that if the outdoor air percentage is reduced it will increase the operative free cooling range, which can add to the energy savings. There is a practical limit to this, however, as many buildings are designed with an outdoor air preheat system that will turn on when outdoor air temperatures are below freezing. With such systems the economizer will be shut off under these conditions. When preheat is applied, significant energy savings can be realized by lowering the outdoor air percentage, since it will directly reduce the load on the preheat coil.
In region “B” the outdoor air damper is automatically adjusted to provide the correct mixture with return airflow to satisfy all of the cooling demand imposed by the return air. Because of this, this is referred to as “free cooling”, since the AHU cooling coil will be off in that mode. However, as the outdoor air percentage is increased the free cooling range will decrease and the added load due to sensible and especially latent heat can become a major factor in total energy costs at higher outdoor air temperatures.
Region “C” of FIG. 1 signifies the range where mechanical cooling is applied and the system is operating at 100% outdoor air. This is also known as the assisted cooling range, where energy savings is realized, even though mechanical cooling is applied, because in this range less cooling is required to condition outdoor air than that for return air.
The boundary between region C and region D is the point at which assisted cooling with outdoor air becomes non-economical due primarily to the latent energy realized at these higher temperatures. With a dry bulb economizer this is typically taken to be 65° F. to 70° F., conditional on geographic location, but can be lower based on the actual return air temperature. However, variations in both return air latent energy, as well as that of outside air, often makes economizers based only on dry bulb or differential dry bulb ineffective, often realizing only a fraction of the potential savings, as well as being potential wasters of energy. In order to be effective, the economizer control function needs to account for latent heat and, when this is done properly, typically using an enthalpy measurement, an optimal switchover point can be provided to yield good energy savings.
Although the differential enthalpy approach can yield the most energy savings, historically, this approach has had several major issues which can lead to energy inefficiency. First, the poor accuracy of most commercial sensors used to provide an enthalpy measurement can result in errors in the economizer logic, causing the economizer to be enabled under inappropriate conditions, as well as not being enabled when it would be beneficial. This is due to stability issues with the hygroscopic materials, such as nylon that have been historically used to form these sensors. (More recently, solid state humidity sensors have become much more reliable, and they can be accurate over a limited range.) These errors are exacerbated by stacking sensor tolerances when a differential enthalpy signal (requiring a separate sensor for both the return air and outside air) is sought after. These errors typically are known to increase as the sensors age.
These errors can be substantially reduced when one utilizes a shared sensor multipoint sampling system to make these measurements. In these systems a sensor, or a single set of multiple sensors, may be used to sense a plurality of locations. For one class of these systems, multiple tubes may be used to bring air samples from multiple locations to a centralized sensor(s). Centrally located air switches and/or solenoid valves may be used in this approach to sequentially switch the air from these locations through the different tubes to the sensor to measure the air from the multiple remote locations. These octopus-like systems sometimes known as star-configured or home run systems use considerable amounts of tubing. An example of such a star-configured system is described in U.S. Pat. No. 6,241,950, which is incorporated herein by reference. Other types of systems known to the art of air monitoring include those that are designed to monitor refrigerants and other toxic gases, which also are star-configured systems.
An exemplary shared sensor multipoint sampling system known as a Networked Air Sampling System, is described in U.S. Pat. No. 6,125,710, which is incorporated herein by reference. Further, U.S. Pat. No. 7,421,911 B2 describes an exemplary duct probe assembly system that can be used in conjunction with a shared sensor multipoint sampling system, such as that described within U.S. Pat. No. 6,125,710, in order to create an enthalpy signal that is precise and not subject to many of the accuracy errors common to discrete enthalpy sensors. The combined teachings of U.S. Pat. Nos. 6,125,710 and 7,421,911 can be applied to greatly improve the performance of a differential enthalpy economizer.
Even with the aforementioned benefits afforded by utilizing a multipoint sampling system, the performance of a differential enthalpy economizer can be further hampered when either the return air or outside air enthalpy measurement become non representative of the cooling load that each would present to the AHU cooling coil. For example, FIG. 2 is a psychrometric chart that illustrates two different conditions where the outside air enthalpy is higher than that of the building's return air. The conditions represented by the outside air and return air states OA#1 and RA#1, respectively, are both conditions that will involve both latent and sensible cooling, because the process of cooling each air source to the shown supply air condition (SA#1) involves both a dry-bulb temperature change, as well as a dewpoint temperature change. Because there is a dewpoint temperature change for each, the cooling process for both RA#1 and OA#1 both result in a “wet-coil” condition. This is because to cool either to the supply air state SA#1, water must be removed from the air. In this case, using enthalpy to determine which air source (OA#1 or RA#1) requires more cooling would be an accurate way to gage whether or not to enable the economizer. In this case, since the return air enthalpy is lower than that of the outside air, the economizer should be disabled. The conditions represented by the outside air and return air states OA#2 and RA#2, respectively, are both conditions that will only involve sensible cooling, because the process of cooling each air source to the shown supply air condition (SA#2 and SA#3) involves only a dry-bulb temperature change. In this particular case, however, even though the enthalpy of the outside air state OA#2 is higher than that of the return air state RA#2 it will actually take less energy to cool the outside air. This is because the energy required to cool OA#2 and RA#2 to the supply air state SA#2 is proportional only to the dry-bulb temperature change. This is referred to as a “dry-coil” condition for each. Therefore, in this case, if enthalpy were used to evaluate each, the economizer would be incorrectly disabled.
There have been attempts to correct for this effect of improperly using differential enthalpy under dry-coil conditions. For example, U.S. Pat. No. 4,312,226 partially corrects for this effect by locking out the economizer if the outdoor air enthalpy is lower than that of the return air, but the outside air temperature is higher than that of the return air. This can be disadvantageous in that it will not enable the economizer under many appropriate dry-coil conditions.
A solid state “electronic enthalpy” controller, such as the H705A made by Honeywell Corporation, provides compensation for dry-coil versus wet coil conditions, based on assumed response curves for combinations of relative humidity and dry-bulb temperature that may be selected based on operating conditions. Each response curve defines the conditions where the economizer will be enabled/disabled and different curves may be selected for different geographic location and supply air conditions. The performance of electronic enthalpy controls such as this are limited in that they provide a fixed curve to which outside air is compared and, they do not account for variations in return air conditions, as well as supply discharge air temperature conditions, which can greatly influence when enabling an economizer will save energy. It is well known to those of ordinary skill in the art of ventilation system design that many ventilation systems are designed around 55 degF supply discharge air conditions. However, as a practical matter, the temperature that the supply air from an AHU is controlled to can be varied up from 55 degF to help reduce heating load during cooler times of the year and down from 55 degF to help improve cooling effectiveness during warmer seasons. As a result, high limit controls based on the fixed curve provided by this type of electronic enthalpy controllers can lead to inefficient economizer operation, due to energy waste that can result when enabling the economizer under incorrect conditions.
It should be further stated that prior art approaches (fixed dry-bulb, differential dry-bulb, fixed enthalpy, differential enthalpy, and electronic enthalpy) to providing an economizer lockout or changeover function are hampered in part by their limitations in accurately representing which airstream (return air or outside air) poses the least total cooling load on the AHU, including both latent and sensible cooling. This is particularly the case during warmer outside air conditions, but it is also hampered by the fact that return air conditions within a building, as well as that of different types of buildings, can vary considerably. For example, high occupant density in a facility can result in significant moisture content within return air, resulting in a wet coil condition (latent cooling) at the AHU, even though outside air conditions would not result in latent cooling. This condition where one air source is subject to latent and sensible cooling while the other is subject to only sensible cooling is particularly problematic, given that even the prior art attempts at compensating for dry-coil versus wet coil conditions (such as for example with the electronic enthalpy approach) would be best suited for conditions where, at a given instance, both air sources would result in the same coil conditions (dry or wet). This, however, is often not the case and it is a disadvantageous aspect of prior art economizer changeover approaches, leading to energy waste.